The present invention relates to a method of manufacturing an acoustic surface wave device which serves as a band pass filter, a delay line or the like.
In general, conversion from acoustic surface waves propagating across a piezoelectric material, such as LiNbO.sub.3 or LiTaO.sub.3, to electrical energy and vice versa is carried out by a transducer which is conventionally composed of a pair of separated, interdigitated comb-shaped electrodes formed on the surface of the piezoelectric material.
One prior art acoustic surface wave device comprises: a piezoelectric substrate having a planer surface for propagation of acoustic surface waves; an input transducer formed on the surface for converting electrical energy into acoustic surface waves; an output transducer formed on the surface and located diagonally with respect to the input transducer, for converting the acoustic surface waves into electrical energy, and; a multistrip coupler (hereinafter referred to as a MSC), including a plurality of parallel and equally spaced conductive elements, formed on the surface and interposed between the input and output transducers so as to be substantially orthogonal to the propagation direction of the acoustic surface waves launched by the input transducer. The MSC serves as an acoustic surface wave path changer only for the acoustic surface waves launched by the input transducer, not for bulk waves which are also launched by the input transducer and travel through the body of the substrate. Thus, the bulk waves which reduce the band-pass performance of the acoustic surface device are prevented from being received into the output transducer. However, this device requires almost twice as much piezoelectric material surface area as a conventional acoustic surface device which has no MSC, and this large surface area results in a higher manufacturing cost.
Another prior art acoustic surface wave device comprises one more input transducer and one more output transducer than the above-mentioned prior art device (see U.S. Pat. No. 3,959,748). In other words, two pairs of transducers, each of which has two connecting pads, are provided. However, only one pair of the transducers which satisfies predetermined conditions, such as time response characteristics and frequency response characteristics, is used. Therefore, in order to select one of the two pairs, testing is carried out which examines whether the transducers satisfy the conditions. First, testing of one pair of transducers is carried out by applying test signals. In this case, four testing probes are placed in contact with the four connecting pads of the pair of transducers. If the first pair of transducers do not satisfy the predetermined conditions, then, testing of the other pair of transducers is carried out in the same way. Once one of the pairs of transducers is selected, the remaining pair of transducers are disabled in order to avoid undesired reflections, which degrade the performance of the device. For example, acoustically absorbent material, such as black wax, is deposited on the remaining pair of tranducers, or dummy impedances are connected across the connecting pads of the remaining pair of transducers. Thus, since either pair of transducers may be usable, the production yield of the acoustic surface devices having two pairs of transducers is improved over that of devices having one pair of transducers located diagonally to each other, which results in a lower mass production cost.
However, in the above-mentioned prior art device having two pairs of transducers, the reliability of testing using a high frequency, such as 30 MHz to 100 MHz, is low because testing can only be carried out by using probing technology which is not suitable for such a high frequency. In addition, during testing, the remaining pair of transducers are not disabled, so that reflections are generated.